It's not possible to search / filter out a specific song from a playlist on the web player. You'll need the desktop client for this. It will also work on mobile when you have Premium.
Check out the Filter section of this article for more information. 🙂
Note: In the desktop client it's possible to find a song by just typing its name too. The client will find the closest match from the playlist. It will play/pause the playback when you hit the spacebar though.
Hope this helps, have a great day!